Powder metal technology is well known to the persons skilled in the art and generally comprises the formation of metal powders which are compacted and then subjected to an elevated temperature so as to produce a sintered product.
Conventional sintering occurs at a maximum temperature of approximately up to 1,150.degree. C. Historically the upper temperature has been limited to this temperature by sintering equipment availability. Therefore copper and nickel have traditionally been used as alloying additions when sintering has been conducted at conventional temperatures of up to 1,150.degree. C., as their oxides are easily reduced at these temperatures in a generated atmosphere, of relatively high dew point containing CO, CO.sub.2 and H.sub.2 /N.sub.2. The use of copper and nickel as an alloying material is expensive. Moreover, copper when utilized in combination with carbon as an alloying material and sintered at high temperatures causes dimensional instability and accordingly the use of same in a high temperature sintering process results in a more difficult process to control the dimensional characteristics of the desired product.
Manufacturers of metal powders utilized in powder metal technology produce pre-alloyed steel powders which are generally more difficult to compact into complex shapes, particularly at higher densities (&gt;7.0 g/cc). Manganese and chromium can be incorporated into pre-alloyed powders provided special manufacturing precautions are taken to minimize the oxygen content, for example, by oil atomization. Notwithstanding this, these powders still have poor compressabilities compared to admixed powders.
Conventional means to increase the strength of powder metal articles use up to 8% nickel, 4% copper and 1.5% molybdenum, in pre-alloyed, partially pre-alloyed, or admixed powders. Furthermore double press double sintering can be used for high performance parts as a means of increasing pan density. Conventional elements are expensive and relatively ineffective for generating mechanical properties equivalent to wrought steel products, which commonly use the more effective strengthening alloying elements manganese and chromium.
Moreover, conventional technology as disclosed in U.S. Pat. No. 2,402,120 teach pulverizing material such as mill scale to a very fine sized powder, and thereafter reducing the mill scale powder to iron powder without melting it.
Furthermore, U.S. Pat. No. 2,289,569 relates generally to powder metallurgy and more particularly to a low melting point alloy powder and to the usage of the low melting point alloy powders in the formation of sintered articles.
Yet another process is disclosed in U.S. Pat. No. 2,027,763 which relates to a process of making sintered hard metal and consists essentially of steps connected with the process in the production of hard metal. In particular, U.S. Pat. No. 2,027,763 relates to a process of making sintered hard metal which comprises producing a spray of dry, finely powdered mixture of fusible metals and a readily fusible auxiliary metal under high pressure producing a spray of adhesive agent customary for binding hard metals under high stress, and so directing the sprays that the spray of metallic powder and the spray of adhesive liquid will meet on their way to the molds, or within the latter, whereby the mold will become filled with a compact moist mass of metallic powder and finally completing the hard metallic particle thus formed by sintering.
U.S. Pat. No. 4,707,332 teaches a process for manufacturing structural parts from intermetallic phases capable of sintering by means of special additives which serve at the same time as sintering assists and increase the ductility of the finished structural product.
Moreover, U.S. Pat. No. 4,464,206 relates to a wrought powder metal process for pre-alloyed powder. In particular, U.S. Pat. No. 4,464,206 teaches a process comprising the steps of communinuting substantially non-compactable pre-alloyed metal powders so as to flatten the particles thereof heating the communinuted particles of metal powder at an elevated temperature, with the particles adhering and forming a mass during heating, crushing the mass of metal powder, compacting the crushed mass of metal powder, sintering the metal powder and hot working the metal powder into a wrought product.
Furthermore various processes have heretofore been designed in order to produce sintered articles having high densities. Such processes include a double press double sintering process as well as hot powder forging where virtually full densities of up to 7.8 g/cc may be obtained. However, such prior art processes are relatively expensive and time consuming.
Other methods to densify or increase the wear resistance of sintered iron based alloys are disclosed in U.S. Pat. 5,151,247 which relates to a method of densifying powder metallurgical parts while U.S. Pat. 4,885,133 relates to a process for producing wear-resistant sintered parts.
Historically steels have been produced with carbon contents of less than 0.8%. However ultrahigh carbon steels have been produced. Ultrahigh carbon steels are carbon steels containing between 0.8% to 2.0% carbon. The processes to produce ultra high carbon steels with fine spheroidized carbides are disclosed in U.S. Pat. 3,951,697 as well as in the article by D. R. Lesver, C. K. Syn, A. Goldberg, J. Wadsworth and O. D. Sherby, entitled "The Case for Ultrahigh-Carbon Steels as Structural Materials" appearing in Journal of the Minerals, Metals and Materials Sot., August 1993.
The processes as described in the prior art present a relatively less cost effective process to achieve the desired mechanical properties of the sintered product.
It is an object of this invention to provide an improved process for producing sintered articles having improved dynamic strength characteristics and an accurate method to control same.
It is a further object of this invention to provide a process for producing sintered articles of densities greater than 7.3 g/cc by a single compaction, single sinter process.
It is a further object of this invention to provide an improved process for producing sintered articles having improved strength characteristics with ultrahigh carbon contents and an accurate method to control same.
The broadest aspect of this invention relates to a process of forming a sintered article using powder metal comprising blending carbon and ferro alloys and lubricant with compressible elemental iron powder, pressing said blended mixture to shape in a single compaction, sintering said article, and then high temperature sintering said article in a reducing atmosphere to produce a sintered article having a high density.
It is another aspect of this invention to provide a sintered powder metal having a composition of between 0.5% to 2.0% manganese, 0.5% to 5.0% molybdenum, 0.1% to 0.35% phosphorous, 0.02% to 0.1% boron, and 0.05% to 0.3% carbon with the remainder being iron and unavoidable impurities, with a sintered density greater than 7.3 g/cc.
It is yet another aspect of this invention to provide a powder metal composition comprising a blend of elemental iron powder, carbon, and ferro manganese, ferro molybdenum, ferro phosphorous, or ferro boron so as to result in an as sintered mass having between: 0.5% to 2.0% manganese; 0.5% to 5.0% molybdenum; 0.1% to 0.35% phosphorous; 0.05% to 0.3% carbon; 0.02% to 0.1% boron or B.sub.4 C; with the remainder being iron and unavoidable impurities.
It is a further aspect of this invention to provide a sintered powder metal article having a composition of between: silicon 0.5% to 1.0%; manganese 0.5% to 2.5%; molybdenum 0% to 2.0%; chromium 0% to 2.0%; phosphorous 0% to 2.0%; carbon 0.8% to 2.0%; remainder being iron and unavoidable impurities and a sintered density of greater than 7.1 g/cc with high ductility.
Moreover it is another aspect of this invention to provide a powder metal composition comprising a blend of elemental iron powder, carbon and ferro silicon, ferro manganese, ferro molybdenum, ferro aluminium, ferro chromium, ferro phosphorous so as to result in an as sintered mass having between: silicon 0.5% to 1.0%; manganese 0.5% to 2.5%; molybdenum 0% to 2.0%; chromium 0% to 2.0%; phosphorous 0% to 0.5%; carbon 0.8% to 2.0%; remainder being iron and unavoidable impurities.
Another aspect of this invention relates to a process of manufacturing a sintered powder metal connecting rod comprising blending carbon and ferro alloys and lubricant with compressible elemental iron powder pressing said blended mixture to shape in a single compaction stage, single sintering said compacted connecting rod, and then high temperature sintering said connecting rod in a reducing atmosphere to produce a sintered powder metal connecting rod having a sintered density of greater than 7.3 g/cc.
Finally, another aspect of this invention relates to a sintered powder metal connecting rod having a density of greater than 7.3 g/cc and composition as follows: